Variable reluctance sensors (VRS) are able to measure the movement or speed of a metal or ferromagnetic object, such as one having teeth or ridges along the object's edge. The movement of the object or object's edges within a magnetic field causes a change in the magnetic flux, inducing a voltage in a conductive coil, which is processed by the sensor. The resulting voltage is proportional to the rate of change in magnetic flux, and speed or position of the object or part of the object may be easily calculated from this voltage measurement. VRS have many applications in automobile control systems, such as measuring wheel speed for anti-lock brakes and wheel bearings and measuring engine revolutions per minute (RPM). Additionally, VRSs may have applications in industrial settings where measuring motor speed or a component speed of rotation is paramount.
A typical VRS may include a permanent magnet with a ferrous pole piece attached to the permanent magnet, while a conductive coil circumnavigates or is wrapped around the ferrous pole piece. The pole piece directs the magnetic field towards a metal rotating gear or other toothed circular component that is ferromagnetic. As the gear's teeth move through the magnetic field, the resulting oscillation in magnetic flux induces a voltage in the conductive coil wrapped around the pole piece. The voltage may be a function of the number of loops the coil makes around the pole piece multiplied by the rate of change of magnetic flux and/or other parameters. The voltage may be measured by a processor connected to the coil, and the processor can calculate the velocity or angular velocity of the gear or other component based on the frequency of the voltage and the known length between gear teeth. VRS are generally low-cost measurement devices, but may be limited from taking accurate measurement of objects moving at slow speeds. This is because the strength of the voltage signal induced depends on the rate of change in the magnetic field, and a lower rate of change (indicating slower speed) translates into a smaller voltage signal.
With VRSs using conventional permanent magnets, significant magnetism will emanate into the far-field, interfering with part of the magnetic field directed towards the gear and thus also interfering with the voltage signal output. The ferrous pole piece orients the magnetic field density closer towards the gear. VRS equipped with a conventional magnet may also require a magnet with strong magnetic force to increase the density of the magnetic field directed towards the gear. Strong magnets may adversely impact other metallic objects nearby, such as credit cards, pacemakers, or other machine parts.